Blow up the moon = what effect on Earth?

I saw Oblvion the other night. All in all, not a horrible movie, although it has some rather large logic holes and questionable science. But it had an intriguing bit of story.

The Antagonists blow up the moon. This, combined with nuclear war to fight them off, ruins the planet.

There are scenes of buildings and bridges that make it clear that New York City is almost totally covered in, what I would assume to be, fall out and silt from the flooding they mention.

Nuclear war aside, just what kind of effect would the Moon’s destruction have on Earth? Let’s also set aside the large lunar fragments that would pound the Earth’s surface at some point. J

I guess, just from a gravitational sense, what would this change for the Earth? Orbit, tides, seasons?

tides would weaken considerably, there would probably be huge earthquakes from the loss of the moon tugging the crust - which could cause areas to sink or rise (like NY in the film)
the orbit wouldn’t change much, the seasons would - the axial tilt of the earth is thought to be in some way caused or accentuated by the moon
the barycenter of the earth-moon-sun system would change a tiny bit but neary enough to do anything
there would be no earth-moon larange points, so some satellites might tumble out of orbit
there would be less tidal drag on the earth - so its rotation would slow down less

Presumably, “blowing up the moon” means some kind of conventional series of explosions?

Aside from the inconceivable amounts of energy involved, a conventional explosion leaves the chunks behind. Few gravitational effects (well, I guess some of those chunks might hit the earth, so some gravitational effects). You’ll still have a roughly moon-massed cloud of stuff with a center of mass where the moon’s center of mass is.

When you blow something up, all the particles are still there. They* don’t just convert to nothingness or energy or what have you. They might not be a solid chunks anymore but the mass is mostly still there.

If, somehow, the remnants of the moon were somehow moved elsewhere (also requiring an inconceivable amount of energy) such that they don’t play a significant role in the earth/moon dynamic, tides would obviously change. You’d still have them but they’d be weaker. Seasons wouldn’t change so much (they’re more a function of the earth’s axial tilt).

Earth’s orbit would change a bit. Compared to the sun, you can model the situation as a single body with mass equal to the sum of the earth and moon’s masses rotating about the barycenter (~center of mass of earth/moon), which is about 1000 miles beneath the surface. Getting rid of the moon’s mass (however you do it) would clearly change the barycenter of the new earth/nothing system and its momentum, which changes the orbital mechanics.

*Well, some of the physical matter does get converted to energy but a vanishingly small portion of it usually.

They didn’t explain how they destroyed the moon in the movie. However it was shown and it was fragmented into pieces, with what looked like several large pieces set for collision with Earth at some point in the future. I’m sure this would of course cause damage.

But forget that particular part. I was aware that the mass would still be there, exerting a similar force. I’m talking about comparing moon vs. no moon.

Without it, what would be different?

I guess I’m curious what effect the moon has on Earth. I remember tides being what is always mentioned from school. But never anything specific (that I remember anyway.)

Are we talking about large scale destruction because of this change?

Well loads of animal species use the moon, full or new, to do the nasty. Also it is a theory that some use it for migratory guides. So blowing it up would screw with a lot of things. Probably see some extinctions as the struggle to adapt to the new environment. Of course that would break up the food chain and that could lead to even more problems.

Tides would definitely be weakened, but I doubt there would be any seismic events as a consequence of the moon disappearing. Right now the crust and mantle are being pulled and churned by the moon every every 24 hours. If that dynamic interaction isn’t producing any earthquakes, I doubt it’s absence would. My understanding is that the effect the moon has on the mantle/crust is fundamentally the same as it’s effect on the oceans, so I’d expect the results to be similar in both cases.

I seem to recall seeing a TV show in which the question of what would happen without a moon.

Ocean tides would be affected, of course. But somehow, and I don’t recall the physics involved, the presence of the relatively large moon somehow affects the earth’s continuing to have approximately the same tilt in relation to its orbit. Without it, the wobble could become extreme, causing huge climate shifts over time.

ETA: Here’s a great link that explains it:

The Master answered this question in 2000.

The moon was allegedly created by a Mars size planet striking the earth with a
glancing or shearing angle. Part of the Mars object merged with earth and the other
part went into an orbit that was much lower than today’s 240,000 miles out. The debris
settled into rings so that for 50 million years more or less the earth had rings like Saturn.

I believe Isaac Asimov wrote a chapter about this theory in one of his many books.
I also believe Asimov wrote about what effects the moon’s absence would have on
a pre-Cambrian earth as well as contemporary earth.

Blowing up the moon wouldn’t cause the chunks to hit the Earth, unless the explosion was violent enough to actually propel them (impart big enough delta-V to lower the orbit to the atmosphere, after which friction would do the rest of the job). Note also that the delta-V has to be in the right direction (retrograde).

That’s a LOT of delta-V. With an explosion violent enough to turn the moon to rubble, perhaps there would be a few bits that hit the earth, but not likely any major hunks. That would take serious energy in the explosion.

So eventually, instead of having a moon, we’d have a little asteroid belt, a bit like the rings of Saturn (only rocky rather than ice). Tides would lessen as the blob of chunks spread out into a circle. Of course, much of the rock would end up being pulled back into the gravitational center, congealing once again into a (smaller) moon.

It’d be fun to see a simulation, and play with variables like how big a bang.

I read one sci-fi store that posited that without the moon’s effect of stripping a tiny bit of atmosphere away, the Earth would have had a much denser atmosphere, so life would be very different. No idea whether that was pure fancy or based in reality.

Didn’t look like anything was on course for collision in Oblivion - just a bunch of really big chunks around the Moon, along with a long string of debris.

I disagree with his reasons for the Moon having a beneficial effect; first, when you consider the size and distance of the Moon from Earth, it actually isn’t a very good asteroid shield. As for tides churning up the ocean, I’d imagine that ocean currents (mainly driven by wind) have a much greater effect. Also, even an unstable axial tilt isn’t necessarily bad for life, although it might be problematic for civilizations that depend on a stable climate, e.g. agriculture (even then, it depends on how fast it changes; a million years gives plenty of time to adapt and isn’t noticeable over multiple generations, doubt the Earth can just flip over overnight, literally speaking).

In some of his stories, Larry Niven used to repeat the factoid that Earth’s atmosphere would have resembled Venus’s if it weren’t for our big moon skimming some off, but in a later annotation to one of those stories he noted that the moon skimmer theory was discredited after he wrote it.

you can do this in game/sim called Universe Sandbox (its pretty fun to play around in anyway)

Just thought I’d mention that in the anime Cowboy Bebop, the moon is cracked in half due to a unfortunate accident with a FTL travel experiment.

Earth is still inhabitable in that universe, but there are occasional “moon fragment falls” that are predicted like weather is now, and people have to periodically evacuate from them.

The major effect in that universe is to make the other colonies in the solar system more important economically and politically… Earth is shown as being practically third-world by the standards of the setting.

Why do dopers obsess on blowing up the moon, knocking it out of orbit, and other ways to mess around with it? Just leave the poor thing alone. Mars is far away, lets blow that up, or one of it’s tiny freaky moons. If we blow up our own moon we won’t have another planet co-orbiting the sun with us (except for Earth II on the other side of the sun).